This invention relates generally to NMR imaging and, more particularly, to NMR imaging employing pulses for time-reversal of nuclear spins to produce spin-echo signals.
Static main magnetic field and RF magnetic field inhomogeneities, i.e., spatial variations in the magnetic field, are a significant problem in NMR imaging, particularly in spin-echo NMR imaging where 180.degree. RF pulses are employed for time-reversing nuclear spins in a plane transverse to the static main magnetic field to produce spin-echo signals. In spin-echo imaging, nuclear spins in a selected region of a sample are nutated away from their alignment with the main magnetic field into a transverse plane by applying to the sample a 90.degree. RF pulse along an axis that is substantially orthogonal to the main magnetic field. Upon being nutated to the transverse plane, the nuclear spins begin to precess in phase in the transverse plane, at the characteristic NMR frequency, .omega., given by the Larmor equation: EQU .omega.=.gamma..beta. (1)
where .gamma. is the gyromagnetic ratio, which is constant for a particular nuclear species, and .beta. is the magnetic field to which the nuclear spins are subjected. However, the nuclear spins quickly lose phase coherence due to local field inhomogeneities and begin to dephase. A 180.degree. RF pulse is then applied to the sample to time-reverse the nuclear spins in the transverse plane, which reverses the net transverse magnetization. Although the resonant, i.e., precession, frequency of the nuclear spins remains the same, the 180.degree. pulse shifts their relative phase by 180.degree. so that the NMR signals produced by the nuclear spins rephase as the nuclear spins regain phase coherence and produce a spin-echo signal. Inhomogeneities in either or both of the main magnetic field and the RF field may prevent the nuclear spins from all being properly time-reversed. Accordingly, instead of all of the nuclear spins regaining phase coherence at the same time, they rephase at different times and produce destructive interference which reduces the amplitude of the desired spin-echo signal.
The two principal sources of magnetic field inhomogeneity are (1) variations in the static main magnetic field due to practical difficulties in constructing magnets capable of producing a perfectly homogeneous field across a sample, and (2) variations in the RF magnetic field due to practical difficulties in constructing RF coils that are capable of producing a perfectly homogeneous RF magnetic field across the sample. As the magnetic field inhomogeneity increases relative to the time-reversing RF magnetic field, the error in rephasing the time-reversed nuclear spins increases. In multiple echo experiments, wherein a plurality of successive 180.degree. pulses are applied to a sample to generate successive spin-echo signals, the errors can produce artifacts in the resulting NMR image and render the image unusable.
Prior attempts to minimize such errors have included the use of an RF field having as large a magnitude as possible, so as to minimize the magnitude of the inhomogeneities relative to the RF field. The difficulty with this approach, particularly when using large static main magnetic fields as is desirable in NMR imaging, is that high-magnitude RF fields can burn a patient and can interfere with metabolic processes in the body. Moreover, as the RF field magnitude is increased, the associated hardware for generating the RF field increases in cost and complexity.
Magnetic field inhomogeneities are not as significant a problem in NMR spectroscopy, wherein the NMR signals from a plurality of different nuclear spins are detected from the whole of a sample in order to analyze the chemical structure of a sample of much smaller sample sizes than are employed in whole body NMR imaging, where sample sizes are comparable to the cross-sectional dimensions of the human body. However, difficulties have been encountered in NMR spectroscopy in producing predetermined rotations of the different nuclear spins in a sample, in part because of the relatively large resonant offsets between the different nuclear species. Recently, it has been proposed to employ composite pulses, comprising sequences of RF pulses for inversion recovery experiments in NMR spectroscopy, as a way of minimizing systematic errors in spin-lattice relaxation time determinations. (See Freeman et al., "Radiofrequency Pulse Sequences Which Compensate Their Own Imperfections", Journal of Magnetic Resonance, Vol. 38, pp. 453-479, 1980.) Composite 180.degree. pulses comprising a pulse for producing a 90.degree. rotation about one axis, a 180.degree. rotation about an orthogonal axis, and a 90.degree. rotation about the first axis have also been investigated in connection with spin-echo experiments in NMR spectroscopy. (See Levitt et al., "Compensation For Pulse Imperfections in NMR Spin-Echo Experiments", Journal of Magnetic Resonance, Vol. 43, pp. 65-80, 1981.) Although such composite pulses have been shown to be capable of minimizing systematic errors and of producing an NMR signal having an amplitude that is substantially preserved, an unfortunate consequence of employing composite pulses for nuclear spin rotations in a transverse plane is that the composite pulses introduce a phase error that varies as a function of the RF and main magnetic field inhomogeneities. Since the inhomogeneities vary spatially throughout the sample, then the phase errors will also spatially vary throughout the sample. If the signals produced by nuclear spins at two different points in a sample happen to have a 180.degree. phase difference, then the signals cancel each other and it appears as if output signals from those points are not produced. Accordingly, composite pulses find applicability in NMR spin-echo spectroscopy only when the magnetic field inhomogeneity is either small or very well defined and the only significant inhomogeneity in the sample is that due to the chemical shift between the different nuclear spins.
In NMR imaging, where the magnetic field inhomogeneities are in general undefined and where rather large sample sizes are employed, rather large undefined phase errors can be encountered using composite pulses, and composite pulses have not been heretofore considered for overcoming time-reversal errors due to magnetic field inhomogeneities.